Host-dependent Eu²⁺/Eu³⁺ co-luminescence and excitation-tunable multicolor emission of Eu-doped AeF₂ (Ae = Ca, Sr, Ba, Ca_0.5Sr_0.5, Ca_0.33Sr_0.33Ba_0.33) single crystals

This study successfully grew a series of mixed-valence Eu²⁺/Eu³⁺ co-activated alkaline-earth fluoride single crystals (CaF₂, SrF₂, BaF₂, Ca_0.5Sr_0.5F₂, and the medium-entropy Ca_0.33Sr_0.33Ba_0.33F₂) using the vertical Bridgman method. During the high-temperature growth process, Eu³⁺ undergoes spontaneous reduction to form Eu²⁺. X-ray photoelectron spectroscopy (XPS) confirmed the coexistence of both valence states, and their relative ratio exhibits a strong host-dependence; for instance, Eu²⁺ is predominant in SrF₂, while Eu³⁺ is dominant in BaF₂. Under ultraviolet excitation, all crystals simultaneously exhibit the broadband emission of Eu²⁺ (400–550 nm) and the narrow-line emission of Eu³⁺ (585–710 nm). The Eu²⁺ emission peak systematically red-shifts from 440 nm in CaF₂ to 485 nm in BaF₂ as the host cation radius increases. Efficient resonant energy transfer from Eu²⁺ to Eu³⁺ is evidenced by spectral overlap and opposing fluorescence lifetime trends. By controlling the host’s composition, the emission chromaticity of the crystals can be continuously tuned from the deep-blue of CaF₂ (0.1720, 0.0382) to the warm-white of BaF₂ (0.4135, 0.3516) under 299 nm excitation. Notably, the medium-entropy Eu: Ca_0.33Sr_0.33Ba_0.33F₂ crystal can achieve a shift in emission tone from neutral white to warm white simply by changing the excitation wavelength. These findings provide a new design strategy for developing single-activator, wide-gamut, tunable luminescent materials for next-generation solid-state lighting and display technologies.